I have found that Tier 2 purple items almost always require a rare purple mat to manufacture. Like yourself I was going to GTN for these mats, but I have recently heard unconfirmed info that if your crew member has a high level of affection for you, they will be more likely to crit on missions and supply more of the rare purple mats. I'm working on this now, but if true its a pretty key component to making Armormech viable for crafting and selling armor items above blue.